This invention relates generally to nuclear reactors, and more particularly to jet pump slip joint seal apparatus for boiling water nuclear reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
Excessive leakage flow can cause oscillating motion in the slip joint, which is a source of detrimental vibration excitation in the jet pump assembly. The slip joint leakage rate can increase due to single loop operation, increased core flow, or jet pump crud deposition. The restrainer bracket laterally supports the inlet mixer through three point contact provided by two set screws and the inlet mixer wedge at an elevation above the slip joint. Set screw gaps can occur during plant operation. Sometimes, the inlet mixer appears to settle to a position away from the set screw, while in other cases, wear occurs between the mixer wedge and the restrainer pad both cases, three point contact is lost and the potential for vibration is significantly increased. Set screw gaps are affected by the difference in thermal and pressure displacements of the shroud, pressure vessel, and rotation of the shroud support plate. In addition to affecting set screw gaps, thermal and pressure displacements of the shroud and the pressure vessel can diminish alignment interaction loads in the jet pump assembly which are beneficial in restraining vibration, such as a lateral force in the slip joint. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue.
High levels of flow induced vibration (FIV) is possible in some jet pump designs at some abnormal operational conditions having increased leakage rates. A labyrinth seal that includes a plurality of circumferential grooves can be added to the exit end of the inlet mixer at the slip joint interface for a new plant design. However, such a modification may be impractical to perform in an operating plant.
It is desirable to provide a jet pump assembly that has a reduced amount of leakage through the slip joint to eliminate high level FIV.
There is therefore provided, in one embodiment of the present invention, a seal apparatus for a jet pump slip joint in a boiling water nuclear reactor pressure vessel. The seal apparatus includes a split seal ring and a segmented diaphragm spring engaging the split seal ring at an inner circumference of the diaphragm spring.
The diaphragm spring includes a plurality of latch assemblies spaced circumferentially around an outer circumference, with each latch assembly configured to engage a diffuser guide ear. A seal ring engagement portion depends from the diaphragm spring and extends around the inner circumference. The seal engagement portion is configured to engage the seal ring. A support portion depends from the diaphragm spring and extends around the outer circumference. A plurality of slots extend from the inner circumference to the support portion of the diaphragm spring. The slots are spaced circumferentially around the inner circumference.
Each latch assembly includes a latch bolt extending through a corresponding latch bolt opening in the support portion of the diaphragm spring. Each latch bolt includes a head and a plurality of ratchet teeth spaced around the periphery of the latch bolt head. A locking spring is coupled to the diaphragm spring. The locking spring is positioned adjacent the latch bolt and configured to engage the ratchet teeth of the latch bolt head. The latch assembly further includes a latch arm having a threaded latch bolt opening. The latch bolt extends through and threadedly engages the latch bolt opening. The latch arm includes a slot sized to receive a diffuser guide ear.
The seal apparatus is installed on the slip joint by positioning the split ring seal and the diaphragm spring on the end of the diffuser with a spring slot engaging each of the diffuser guide ears. The latch bolt of each latch assembly is tightened so that the latch arm swings into position and engages a corresponding guide ear. The inlet mixer is installed through the split ring seal and diaphragm spring and into the diffuser to form the slip joint. The latch bolt is tightened further to capture the latch arm slot against the guide ear to engage the seal ring with the seal engagement portion of the diaphragm spring. The latch bolt is locked in place by the locking spring engaging the ratchet teeth of the clamp bolt head.
The above described seal apparatus restricts leakage flow between the inlet mixer and the diffuser at the slip joint to prevent oscillating motion and to eliminate high level flow induced vibration. Additionally, the wedging action of the seal in the slip joint opening provides a rigid resistance to oscillating motion.